The operation of any scanning system requires that the successive radiation signals from a given pixel in the viewed scene be converted into an output signal representing that particular pixel. In a line array system, this may be accomplished by a direct, synchronized transfer and conversion of each pixel input signal to a corresponding output signal available to the observer.
This transfer may require integration of successive signals from a given pixel prior to electronic transfer of the pixel output. The common method used for such integration is the use of a semiconductor charge transfer device (CTD) array, such as a charge-coupled device (CCD) array, or a bucket-brigade (BB) array. Background noise is a substantial problem, however, when employing a two-dimensional focal plane as contrasted to a line scanner in which the signal is proportional to changes in intensity as the scene sweeps across the detector. Many of the prior art difficulties in providing adequate photo-detector signals from a two dimensioned focal plane have been solved by the "Z-technology" developed by the assignee of this application. The term "Z-technology" refers to the fact that a focal plane module, which has circuit carrying layers perpendicular to the focal plane, has a depth dimension Z in addition to the X and Y dimensions of the focal plane.
Maximum image enhancement, such as would be required to observe an image at maximum range, requires the smallest instantaneous field of view (IFOV). The minimum IFOV is equivalent to one pixel which is represented by the area of object space observed by one detector at a given time. Utilizing Z-technology a large number of detectors can be assembled to observe a large search volume comprising a plurality of focal planes, such as would be required in an aircraft scanning and tracking system. Such systems, however, require enormous data processing capabilities which are beyond the capabilities of most, if not all, data processing equipment currently available. Presently available scanning and tracking systems, particularly those using Z-technology, use various methods to compress or reduce the amount and rate of data output in order effect data processing. For example, one such method is the so-called "thresholding" technique in which only those pixels containing targets or target-like objects are passed to the data processor. However, if the thresholding is accomplished with high sensitivity, most of the background noise passes through the filter to the data processor. On the other hand, if threshold sensitivity is reduced small targets become lost in the background clutter.